Early on June 25, the SEAL divers entered the floodwaters at Sam Yaek and found that the passage opening was much smaller than they realized and was clogged with rocks and mud. Even after widening the opening, they still had to remove their tanks and push them ahead of them to fit through the opening. The current was also incredibly strong, making progress difficult and risky. The SEALS were experienced in open water diving, but none of them had dived in caves before.

Cave diving is more dangerous than open water diving because the surface isn’t readily reachable when one is miles deep in cave passages. Caves pose many risks, such as tight squeezes that can snag gear, and complete darkness that makes progress without a flashlight impossible. In addition, it’s easy for divers to become disoriented in twisting passages without light from the surface to differentiate up from down, so laying a guideline is important to keep divers on the correct path. The SEALs weren’t familiar with these risks and didn’t know to use a guideline. They made it past Sam Yaek to a higher area of dry ground but saw another flooded passage. Their air supply wasn’t sufficient to continue forward, so they had to return.